                Solifenacin succinate (I) is chemically known as (1S)-3,4-dihydro-1-phenyl-2(1H)-isoquinoline carboxylic acid (3R)-1-azabicyclo[2.2.2]oct-3-yl ester succinate.        
Solifenacin is a potent muscarinic M3 receptor antagonist. Muscarinic receptors play an important role in several major cholinergically mediated functions, including contractions of the urinary bladder, gastrointestinal smooth muscle, saliva production, and iris sphincter function. Solifenacin has greater affinity for the M3 receptor than for the other known muscarinic receptors. Solifenacin succinate is commercially available under the brand name Vesicare®. It has been approved for the treatment of overactive bladder with symptoms of urge urinary incontinence, urgency, and urinary frequency.
Solifenacin (II) and its pharmaceutically acceptable salts are disclosed in U.S. Pat. No. 6,017,927.
According to the process disclosed in U.S. Pat. No. 6,017,927, Solifenacin may be prepared by condensation of (1S)-1-phenyl-1,2,3,4-tetrahydroisoquinoline of formula (III) with ethyl chloroformate to produce (1S)-ethyl 1-phenyl-1,2,3,4-tetrahydro-2-isoquinolinecarboxylate of formula (IV), which is further reacted with (R)-(−)-3-quinuclidinol of formula (V) to produce Solifenacin (II). Solifenacin is converted to Solifenacin hydrochloride by treating with HCl.

The major disadvantage with the above process is that it involves the use of expensive optically pure R-(−)-3-quinuclidinol. Further, the above process suffers from low yield and low optical purity of Solifenacin.
US '927 also discloses another variant process, wherein (1S)-1-phenyl-1,2,3,4-tetrahydroisoquinoline (III) is reacted with 3-quinuclidinyl chloroformate monohydrochioride (VI) to produce diastereomeric mixture of Solifenacin (IIa) ([(1S,3′R)-isomer] and (1S,3′S)-isomer).

However, the above process is silent about the separation of Solifenacin from the diastereomeric mixture of Solifenacin (IIa) ([(1S,3′R)-isomer] and (1S,3′S)-isomer).
The U.S. Pat. No. 7,829,715 B2 discloses a process for the condensation of racemic ethyl 1-phenyl-1,2,3,4-tetrahydro-2-isoquinolinecarboxylate (IVa) with optically pure (R)-(−)-3-quinuclidinol (V) to produce diastereomeric mixture of Solifenacin (IIb) [(1S,3′R)-isomer and (1R,3′R)-isomer], which is further treated with succinic acid in ethanol and ethyl acetate solvent mixture to produce optically more Solifenacin succinate (I).

The above process also involves the use of expensive optically pure R-(−)-3-quinuclidinol. Hence, there is need to provide a simple and cost effective process for Solifenacin succinate. The process of the present invention avoids the use of very expensive optically pure R-(−)-3-quinuclidinol.
The present invention specifically directed towards the process for the preparation of Solifenacin succinate (I), by condensation of (1S)-alkyl 1-phenyl-1,2,3,4-tetrahydro-2-isoquinolinecarboxylate (IVb) with (RS)-3-quinuclidinol (Va) followed by selective isolation of optically pure Solifenacin succinate of formula (I) by diastereomeric crystallization using succinic acid.